Abstract Human decision-making is thought to involve the interplay of two distinct information processing systems: a rational (logical) system and an experiential (intuitive) system ( Epstein, 1994). Moreover, the ability to engage in rational processing is believed to be constrained by working memory capacity (WMC) ( Feldman Barrett, Tugade, & Engle, 2004). Accordingly, preference for rationality, but not preference for experientiality, was expected to mediate the relationship between WMC and performance on cognitive tasks that require logical reasoning. Path analysis using AMOS 18, with data from 269 non-paired twins, confirmed this mediation hypothesis. Higher WMC was predictive of stronger preference for rationality, which, in turn, was predictive of better syllogistic reasoning, lower susceptibility to gambling biases, and lower superstitiousness and categorical thinking. As expected, WMC was unrelated to preference for experiential processing, and higher experientiality predicted poorer performance on the syllogistic reasoning task, higher susceptibility to gambling biases and greater superstitiousness.